Through air drying has become the technology of preference for making one-ply absorbent paper for many manufacturers who build new absorbent paper machines as, on balance, through air drying ("TAD") offers many economic benefits as compared to the older technique of conventional wet-pressing ("CWP"). With through air drying, it is possible to produce a single-ply absorbent paper in the form of a tissue with good initial softness and bulk as it leaves the absorbent paper machine.
In the older wet pressing method, to produce a premium quality, absorbent paper, it has normally been preferred to combine two plies by embossing them together. In this way, the rougher air-side surfaces of each ply may be joined to each other and thereby concealed within the sheet. However, producing two-ply products, even on state of the art CWP machines, lowers paper machine productivity by about 20% as compared to a one-ply product. In addition, there may be a substantial cost penalty involved in the production of two-ply products because the parent rolls of each ply are not always of the same length, and a break in either of the single plies forces the operation to be shut down until it can be remedied. Also, it is not normally economic to convert older CWP tissue machines to TAD. But even though through air drying has often been preferred for new machines, conventional wet pressing is not without its advantages as well. Water may normally be removed from a cellulosic web at lower energy cost by mechanical means such as by overall compaction than by drying using hot air.
What has been needed in the art is a method of making a premium quality single-ply absorbent paper using conventional wet pressing having a high bulk and excellent softness attributes. In this way advantages of each technology could be combined so older CWP machines can be used to produce high quality single ply absorbent paper products in the form of bathroom tissue and facial tissue at a cost which is far lower than that associated with producing two-ply absorbent paper.
Among the more significant barriers to the production of single-ply CWP absorbent paper have been the thinness and the extreme sidedness of single-ply webs. An absorbent product's softness can be increased by lowering its strength, as it is known that softness and strength are inversely related. However, a product having very low strength will present difficulties in manufacturing and will be rejected by consumers as it will not hold up in use. Use of premium, low coarseness fibers, such as eucalyptus, and stratification of the furnish so that the premium softness fibers are on the outer layers of the tissue is another way of addressing the low softness of CWP products; however this solution is expensive to apply, both in terms of equipment and ongoing fiber costs. In any case, neither of these schemes addresses the problem of thinness of the web. TAD processes employing fiber stratification can produce a nice, soft, bulky sheet having adequate strength and good similarity of the surface texture on the front of the sheet as compared to the back. Having the same texture on front and back is considered to be quite desirable in these products or, more precisely, having differing texture is generally considered quite undesirable. Because of the deficiencies mentioned above, many single-ply CWP products currently found in the marketplace are typically low end products. These products often are considered deficient in thickness, softness, and exhibit excessive two sidedness. Accordingly, these products have had rather low consumer acceptance and are typically used in "away from home" applications in which the person buying the tissue is not the user.
We have found that we can produce a soft, high basis weight, high bulk, high strength CWP bathroom tissue, facial tissue, and napkins with low sidedness having a serpentine configuration by judicious combination of several techniques as described herein. Basically, these techniques fall into four categories: (a) providing a furnish to a web such that at least 20 percent by weight of the fibers in the web have a coarseness exceeding 23 mg/100 m; (b) at least about 20 percent by weight of the fibers in the web have a coarseness of less than about 12 mg/100 m; (c) the weight average coarseness to length ratio of the fibers in the web is less than about 8.5 mg/100 m/mm; and (d) optionally, the weight-weighted average fiber length is selected to be greater than about 1.75 mm. In addition, optionally, a controlled amount of temporary wet strength may be added along with a softener or debonder. By various combinations of these techniques as described, taught, and exemplified herein, it is possible to almost "dial in" for the absorbent paper the required degree of softness, bulk, and strength depending upon the desired goals. The use of softeners having a melting range of about 1.degree.-40.degree. C. and being dispensable at a temperature of about 1.degree.-100.degree. C. suitably 1.degree.-40.degree. C. preferably 20.degree.-25.degree. C. further improves the properties of the one-ply, high bulk, soft, absorbent paper product having a serpentine configuration.
1. Field of the Invention
The present invention is directed to a soft, strong in use, bulky single-ply absorbent paper product having a serpentine configuration and processes for the manufacture of such paper. More particularly, this invention is directed to a soft, strong-in-use, bulky, single-ply bathroom tissue, facial tissue, and napkin.
2. Description of Background Art
Paper is generally manufactured by suspending cellulosic fiber of appropriate geometric dimensions in an aqueous medium and then removing most of the liquid. The paper derives some of its structural integrity from the mechanical arrangement of the cellulosic fibers in the web, but most by far of the paper's strength is derived from hydrogen bonding which links the cellulosic fibers to one another. With paper intended for use as bathroom tissue, the degree of strength imparted by this inter-fiber bonding, while necessary to the utility of the product, can result in a lack of perceived softness that is inimical to consumer acceptance. One common method of increasing the perceived softness of bathroom tissue is to crepe the paper. Creping is generally effected by fixing the cellulosic web to a Yankee drum thermal drying means with an adhesive/release agent combination and then scraping the web off the Yankee by means of a creping blade. Creping, by breaking a significant number of inter-fiber bonds adds to and increases the perceived softness of resulting tissue product.
Another method of increasing a web's softness is through the addition of chemical softening and debonding agents. Compounds such as quaternary amines that function as debonding agents are often incorporated into the paper web. These cationic quaternary amines can be added to the initial fibrous slurry from which the paper web is subsequently made. Alternatively, the chemical debonding agent may be sprayed onto the cellulosic web after it is formed but before it is dried.
The most pertinent prior art patents will be discussed but, in our view, none of them can be fairly said to apply to the one-ply, absorbent paper of this invention which exhibits high bulk, soft and strong attributes. U.S. Pat. Nos. 5,405,499; 5,585,685; and 5,679,218 are irrelevant to our invention since, by the processes disclosed in those applications, the high coarseness fibers necessary to practice our invention are excluded.
Other prior references include Williams, U.S. Pat. No. 4,247,362, which is related to non delignified softwood and specially treated defibered hardwood; the majority of fibers in the sheet are softwood; Cochrane, et al., U.S. Pat. No. 4,874,465 discloses a sliced (lengthwise) fiber; Reeves, et al., U.S. Pat. No. 5,320,710 discloses hesperaloe fiber; Back, et al., U.S. Pat. No. 5,582,681 discloses newsprint printed with oil-containing ink wherein the pulp is treated with enzymes. All of these patents require the use of unique specialized fiber or a non-conventional stock preparation method, in contrast to the current invention which utilizes conventional paper making fibers prepared by standard pulping and stock preparation methods. Representative layered or stratified paper products in contrast to the present invention which comprises a single (homogenous) layer include Dunning et al, U.S. Pat. No. 4,166,001; Carstens, U.S. Pat. No. 4,300,981; Awofeso, et al., U.S. Pat. No. 5,087,324; and Awofeso, et al., U.S. Pat. No. 5,164,045. From the foregoing discussion of the prior art, it is clear that none of the references relate to one-ply, absorbent papers produced by (a) providing a furnish to a web such that at least 20 percent by weight of the fibers in the web have a coarseness exceeding 23 mg/100 m; (b) at least about 20 percent by weight of the fibers in the web have a coarseness of less than about 12 mg/100 m; (c) the weight average coarseness to length ratio of the fibers in the web is less than about 8.5 mg/100 m/mm; and (d) optionally, the weight-weighted average fiber length is selected to be greater than about 1.75 mm.
In addition, the foregoing prior art references do not disclose or suggest a high-softness, bulky, strong one-ply absorbent paper product in the form of a bathroom tissue and facial tissue having serpentine configuration and having a total specific tensile strength of no more than 200 grams per three inches per pound per 3000 square foot ream, a cross direction wet tensile strength of at least 2.75 grams per three inches per pound per 3000 square foot ream, a specific geometric mean tensile stiffness of 0.5 to 3.2 grams per inch per percent strain per pound per 3,000 square foot ream, a GM friction deviation of no more than 0.25 which are produced when, optionally, temporary wet strength agents and softeners/debonders are added to the web or furnish after the fiber selection has been made wherein (a) at least 20 percent by weight of the fibers in the web have a coarseness exceeding 23 mg/100 m; (b) at least about 20 percent by weight of the fibers in the web have a coarseness of less than about 12 mg/100 m; (c) the weight average coarseness to length ratio of the fibers in the web is less than about 8.5 mg/100 m/mm; and (d) optionally, the weight-weighted average fiber length is greater than about 1.75 mm.